1. Field of the Invention
The present invention relates to a method for processing a liquid nitrogen-rich organic waste product to an aqueous solution, in which method the waste product is subjected to a biological conversion process so as to obtain a biologically substantially stable aqueous solution, which conversion process includes at least a nitrification step wherein nitrifiable ammonium nitrogen from the waste product is converted into nitrate nitrogen in a nitrification reactor by means of nitrifying bacteria, in the event the nitrate content in the so-obtained nitrified fraction would exceed a predetermined nitrogen content, the nitrate content is limited to a content at which the nitrifying bacteria are active by subjecting at least a portion of a previously nitrified fraction to a denitrification step and subsequently to the nitrification step together with a further fraction which is to be nitrified.
The expression "liquid nitrogen rich organic waste product" used herein refers in particular to liquid manure products as obtained directly from farms, for example semi-liquid manure, liquid manure and the like, but possibly also further processing products of these manure products such as for example the aqueous fraction obtained after removal of the solid fraction out of semi-liquid manure or the resulting products obtained after a partial fermentation of the manure product, for example for the production of biogas. Of course, also manure products of human origin are included in this expression. Further the expression also embraces other liquid nitrogen rich organic waste products such as for example waste water from composting installations.
2. The Prior Art
EP-A-0 423 889 in the name of one of both applicants discloses already a method for processing semi-liquid manure or fermented semi-liquid manure of the hereabove indicated type. In this known method, use is made of a water purification installation wherein the manure product is subjected successively to a nitrification step and subsequently to a denitrification step so as to obtain a purified aqueous solution wherein the nitrogen and the organic matter are eliminated as much as possible since the denitrified fraction is subsequently discharged.
In order to eliminate the nitrogen from the manure as completely as possible, a portion of the denitrified fraction is recycled to the nitrification reactor in such a manner that the nitrate content in this reactor is preferably limited to between 1000 and 1400 mg NO.sub.3.sup.- -N/l. Within these limits, an optimal conversion of ammonium to nitrate nitrogen is obtained. For the same reason, use is further made of methanol as necessary carbon and energy source for the denitrifying bacteria. When use would be made herefor of said manure product, ammonium nitrogen is again produced during the denitrification which nitrogen would therefore end in the effluent of the purification installation. In the method according to this European patent application, purified waste water is obtained as effluent from the denitrification reactor.
Such a method has, however, the drawback that all of the plant nutrition elements are lost. Moreover, such a purification is never complete so that an amount of polluents will always arrive in the surface waters. In order to recuperate a portion of these nutrient elements, the solid fraction can be separated in advance out of the manure and can be dried, which requires however a lot of energy. In this case, the liquid fraction remains moreover to be purified and has still to be discharged.
An object of the present invention is therefore to provide a new method for processing liquid nitrogen rich organic waste products which obviates the hereabove indicated drawbacks and which permits in particular the nutrient elements present in this waste product to be used in a useful way.
The method according to the invention is characterized to this end in that said liquid nitrogen rich waste product is processed to an aqueous fertilizer solution which is separated off out of said nitrification reactor after the nitrification step and said denitrification step for limiting the nitrogen content is only performed if the fraction to be nitrified has a content of nitrifiable nitrogen which is higher than a predetermined maximum nitrogen content of at least 3000 mg nitrifiable nitrogen per liter, in which case the nitrate content in the nitrified fraction is limited by said denitrification step to a content of between 1500 mg NO.sub.3.sup.- -N/l and said predetermined maximum nitrogen content.
In contrast to the method according to EP-A-0 423 889, an aqueous solution is separated off in the method according to the invention after the nitrification step, more particularly an aqueous fertilizer solution containing plant nutrition elements including nitrates. A denitrification step for limiting the nitrogen content is only performed if this nitrate content after the nitrification would exceed a certain maximum value at which the nitrate concentration would hamper the biological processes too strongly. Indeed, the plant nutrition elements including nitrogen are preferably retained as much as possible.
Due to the fact that the obtained product is biologically substantially stable, it can be added for example to a nutrient solution for a hydroponic culture without bringing about aerobic biological processes which would withdraw too much oxygen which is essential for the plants from the nutrient solution. Further, the nitrification step is also important. Indeed, the nitrogen present in the waste product mainly in the form of ammonium is converted in this step into nitrate nitrogen. In a hydroponic culture, the presence of ammonium has an inhibiting influence on the uptake of other cations such as potassium. A high ammonium concentration is moreover even harmful for the plants. Since the waste product, such as for example semi-liquid manure, has a high ammonium-content, it is consequently clear that such a product cannot be used as such in a hydroponic culture, even not when the solid fraction has been removed or when the organic matter has been decomposed moreover in a conventional aerobic water purification installation or in a fermentation installation. Indeed, in these latter cases the ammonium content remains still too high. A sufficiently far-reaching nitrification is further also important in order that the fertilizer solution will be substantially free from nitrites, since nitrites are even more harmful for plants than ammonium.
An important advantage of the method according to the invention consists in that it allows to convert a nitrogen rich organic waste product into a valuable fertilizer solution which is in particular suited for feeding plants in an hydroponic system or for being used as leaf nutrition. By using such a waste product, an important saving as to raw materials such as inorganic salts which are normally used for producing the fertilizer solutions, can be realized. Further, a saving of energy is realized since the manufacture of synthetic fertilizers requires a large amount of energy. The method according to the invention requires on the other hand only a little amount of energy since the nutrient elements present in the waste product are converted mainly by biological processes into a form absorbable by the plant.
In the method according to DE-C-3920539 for processing semi-liquid manure, this manure is also first nitrified, just as in EP-A-0 423 889, and subsequently denitrified in order to eliminate therefrom an amount of the nitrogen present therein. The so obtained denitrified product is however not a fertilizer solution but has still the same applications as the original semi-liquid manure and can clearly not be used in hydroponic systems or as leaf nutrition. Indeed, besides an amount of solid organic matter, the obtained product further still contains an important amount of ammonium and possibly even of nitrite due to the large amount of organic matter, the oxidation of which will consume much oxygen during the nitrification step.
DD-A-154693 discloses also a method for processing semi-liquid manure. In this known method, the solid matter is separated first out of the manure. The resulting liquid is then denitrified and nitrified, and the nitrified fraction is recycled again in this purification process. A portion of the nitrified fraction is namely recycled to the denitrification reactor while the remaining portion is added to the semi-liquid manure in the stable so as to obtain a prior denitrification. The ratio between both portions is determined in function of the pH changes in both reactors and in function of the amount of nitrate which is required in the denitrification reactor to decompose the organic matter. Due to the very high content of organic matter, a large portion of the nitrogen will consequently be eliminated in the denitrification reactor whereas the remaining organic matter in the influent to the nitrification reactor prevents the nitrifying bacteria from converting all of the nitrifiable nitrogen completely into nitrate so that the nitrified fraction will comprise a relatively high amount of ammonium and even of nitrite.